A team of astronomers has found the first Earth-sized planet outside our solar system that has a rocky composition like Earth’s. This exoplanet, known as Kepler-78b, orbits its star closely every 8.5 hours, making it much too hot to support life.

This Earth-sized planet was discovered using data from NASA’s Kepler Space Telescope and confirmed and characterized with the W. M. Keck Observatory.

Every 8.5 hours, the planet passes in front of its host star, blocking a small fraction of the starlight. Researchers picked up these telltale dimmings while analyzing the Kepler data.

The team, led by Andrew Howard from the Institute for Astronomy at the University of Hawaii at Manoa, then measured the mass of the planet with the Keck Observatory on Mauna Kea in Hawaii. Using the 10-meter Keck I Telescope fitted with the HIRES instrument, the team employed the radial velocity method to measure how much an orbiting planet causes its star to wobble and to determine the planet’s mass. This is another excellent example of the synergy between the Kepler survey, which has identified more than 3,000 potential exoplanet candidates, and the Keck Observatory, which plays a leading role in conducting precise Doppler measurements of the exoplanet candidates.

A handful of planets the size or mass of Earth have been discovered recently. This is the first one with both quantities measured. "When you have both the size and the mass of an object, you can calculate its density and thereby determine its composition," said Howard.

With a radius about 1.2 times that of Earth and a mass equal to about 1.7 times Earth’s, Kepler-78b has a density the same as Earth’s, suggesting that it is also made primarily of rock and iron. Its star is slightly smaller and less massive than the Sun and is located about 400 light-years from Earth in the constellation Cygnus.

Kepler-78b is a member of a new class of "ultra-short period" planets recently identified by the Kepler spacecraft. These newfound worlds all orbit their stars with orbital periods of less than 12 hours. They’re also small, about one to two times the size of Earth. Kepler-78b is the first planet in this new class to have its mass measured. It is a mystery how these planets formed and made it so close to their host stars — only 1 percent of the Earth-Sun separation in the case of Kepler-78b.

A companion study led by Francesco Pepe from the University of Geneva in Switzerland used the same Kepler data but independent radial velocity observations.

The two studies found similar results. "The gold standard in science is having your findings reproduced by other researchers," said Howard. "In this case, we did not have to wait for this to happen."